(a) Field of the Invention
This invention relates to the recovery of petroleum from subterranean reservoirs, and more particularly, to an enhanced oil recovery process especially suitable for heterogeneous reservoirs.
(B) Description of the Prior Art
It has long been recognized that substantial amounts of oil remain unrecovered at the completion of normal primary recovery operations. In order to improve the recovery of oil, it is common practice to supplement the native reservoir energy by various enhanced recovery means including the use of various aqueous fluids to displace oil from a formation.
It is known to use small slugs of a dilute aqueous alkaline alkali metal silicate solution in various enhanced oil recovery processes. U.S. Pat. Nos. 3,805,893 issued Apr. 23, 1974 and 3,871,452 and 3,871,453 issued Mar. 18, 1975 involve, in part, injection into a subterranean petroleum reservoir of small slugs of a dilute aqueous solution containing from about 0.1 to 0.8 weight percent of an alkaline alkali metal silicate having a molar ratio of M.sub.2 O/SiO.sub.2 of 1 or above, wherein M is an alkali metal atom. It is suggested that the solution be injected for from 1 hours to 7 days at a rate of from 100 to 2,000 barrels per day per well. U.S. Pat. No. 3,920,074 issued Nov. 18, 1975 suggests the use of 0.005 to 0.8 weight percent of the same silicate material. It is further suggested that the composition be injected at the rate of about 0.1 to 10 barrels/foot of vertical formation thickness. These patents teach the use of the composition to reduce oil saturation in the formation and, in conjunction with an agent such as calcium chloride which reacts with the silicate material, to form plugging precipitates as part of an oil recovery process. In the various patents, the aqueous solution of the silicate may be injected alone or as an ingredient of an emulsion. In some patents the aqueous silicate solution is followed by a water drive. In all instances the aqueous silicate solution is used in slugs having a relatively small volume.
While these processes have utility in recovering additional quantities of oil from subterranean petroleum reservoirs, nevertheless, it is desirable to even further increase the amount of oil recoverable from these reservoirs.
Accordingly, a principal object of this invention is to provide an improved process for enhanced oil recovery from a subterranean reservoir.
Another object of the invention is to provide an improved process for enhanced oil recovery from a heterogeneous subterranean formation.
A further object of the invention is to provide such a process wherein channeling of the injected enhanced recovery fluid is reduced.
A still further object of the invention is to provide such a process which is especially suitable for recovering oil having a relatively high acid number.
Yet another object is to provide such a process wherein a preflush is injected to remove divalent cations from the portion of the reservoir to be contacted by the enhanced recovery fluid.
Other objects, advantages and features will be apparent from the following description.